This invention relates generally to resistor fabrication and, more specifically, to highly resistive structures formed on a flexible base material and techniques for fabricating such a structure.
A number of commercial applications implement semiconductor devices which may be susceptible to electrostatic discharge. One such device is a semiconductor light emitting diode (LED). LEDs are semiconductor chips that are packaged to emit radiation in response to an applied voltage or current. These LEDs are used in a number of commercial applications such as automotive, display, safety/emergency and directed area lighting. LEDs may be fabricated using any materials which emit visible, ultraviolet or infrared radiation. Thus, LED devices may comprise materials having a p-n junction of semiconductor layers capable of emitting the desired radiation. For example, the LED devices may be fabricated using any desirable III-V semiconductor compound layer, such as GaAs, GaAlAs, GaN, InGaN, GaP, etc., II-VI semiconductor compound layers, such as ZnSe, ZnSSe, CdTe, etc., or IVxe2x80x94IV semiconductor compound layers, such as SiC, for example. Further, the LED devices may also include other layers such as cladding layers, wave guide layers and contact layers.
Currently semiconductor devices, such as LEDs, may be fabricated on a flexible base material such as polyimide to provide flexible device arrays that are conformal to a variety of different shapes for use in a variety of products. For instance, LED arrays may be fabricated on a flexible base material for use in lighting products such as round lamp bulbs, flood lights, cylindrical flashlights, etc. However, semiconductor devices such as GaN based LEDs, for example, are sensitive to electrostatic discharge, because they are fabricated on insulating sapphire substrates. As a result, there is no clear discharge path for electrostatic discharge other than through the LED. Electrostatic discharge through the LED may result in severe damage to the LED by degrading the p-n junction characteristics of the device. It is therefore advantageous to create an alternative path for electrostatic discharge.
One mechanism for minimizing the likelihood of electrostatic damage to the semiconductor devices, such as LEDs, is to incorporate a high value bleed resistor to provide a discharge path for charge dissipation. However, for semiconductor devices fabricated on flexible base materials, the fabrication of resistors may offer certain challenges. It is often difficult to fabricate small high value thin film resistors in the range of 100 kohms-1 Mohm on a flexible base material such as polyimide. Commonly used resistor films such as tantalum nitride (TaN2), nickel chrome (NiCr), and chromium silicide (CrSi) have sheet resistance values in the range of 100-300 ohms/square. At these values, resistors in the range of 100 kohms-1 Mohm are generally patterned in serpentine structures which may cover very large areas. For semiconductor devices fabricated on a flexible base material, the large serpentine structures used to fabricate the highly resistive thin film resistors may be unfeasible. Further, processing on flexible based materials, such as polyimide films, generally require relatively low temperatures ( less than 200xc2x0 C.) such that the film maintains structural integrity. The lower temperatures required for processing on flexible base materials precludes the use of many of the materials that are typically used to fabricate high value resistors in semiconductor devices, since the materials require processing temperatures greater than 200xc2x0 C.
In accordance with one aspect of the present invention, there is provided a method of fabricating a resistor comprising the acts of activating a region on the surface of a flexible substrate, thereby forming an activated region, forming a resistive layer in the activated region, depositing one or more interconnect layers over at least a portion of the resistive layer, and patterning the one or more interconnect layers to form terminals of a resistor.
In accordance with another aspect of the present invention, there is provided a method of fabricating a resistor comprising the acts of activating the surface of a flexible substrate, thereby forming an activated layer, depositing a first metal layer over the surface of the activated layer, thereby causing a reaction in the activated layer that results in the formation of a resistive layer, depositing one or more interconnect layers over the first metal layer, etching each of the one or more interconnect layers and the first metal layer to the resistive layer, thereby forming terminals, and patterning the resistive layer to form a resistor coupled between the terminals.
In accordance with a further aspect of the present invention, there is provided a method of fabricating a resistor comprising the acts of depositing a masking layer over the surface of a flexible substrate, forming an opening in the masking layer, thereby exposing a portion of the flexible substrate through the opening, activating the exposed portion of the flexible substrate, thereby forming an activated region, removing the masking layer from the surface of the flexible substrate, depositing one or more interconnect layers over the surface of the flexible substrate, and patterning the one or more interconnect layers to form isolated terminals electrically coupled with respect to each other by the activated region.
In accordance with still another aspect of the present invention, there is provided a device comprising a flexible substrate, a resistive region formed on the flexible substrate and having a first end and a second end, and conductive terminals coupled to each of the first end and the second end.
In accordance with yet another aspect of the present invention, there is provided a device comprising a flexible substrate having a first side and a second side, a light emitting diode (LED) coupled to the first side of the flexible substrate and electrically coupled to contact regions on the second side of the flexible substrate, and a resistor formed on the second side of the flexible substrate, wherein the resistor is electrically coupled between each of the contact regions.